The present invention relates in general to a battery pack for night vision goggles, and in particular to an EMI-hardened battery pack that has a shunt circuit for protection against high-voltage RF signals.
Night vision goggles are well known for their ability to enhance night-time vision. The goggles employ intensifier devices which operate to multiply the incident light received to produce a display that is bright enough for presentation to the eyes of a viewer. For instance, the U.S. military uses night goggles during night-time operations to sight objects that otherwise would not be visible.
A typical arrangement for a night vision goggle system is shown in FIG. 1. The night goggles 1 are attached to the front of a helmet 2 such that they can be viewed by an operator 3. A battery pack 10, which provides the night goggles 1 with operating potential, is attached to the rear of the helmet 2. A light emitting diode (LED) 4, located on the night goggles 1 in clear view of the operator 3, indicates when the voltage provided by the battery pack 10 is at a low level. The voltage is supplied from the battery pack 10 to the night goggles 1 by a first conductor 5. The LED 4 is connected to a low battery indicator in the battery pack 10 by second and third conductors 6 and 7.
The night vision goggles 1 are routinely subjected to high RF fields (EMI radiation) in excess of several hundred volts/meter when used in military aircraft. These fields can be internally generated and/or be externally generated. Because the first, second and third conductors 5, 6 and 7 extend from the back of the helmet 2, around the side of the helmet and to the front of the helmet 2, they are exposed to these very high RF fields. The conductors 5, 6 and 7 behave as antennae and pick up these RF signals. When transmitted to the battery pack 10, the RF signals tend to affect the operation of the battery pack 10 and which, in turn, affects the operation of the night goggle system. These RF signals are of large magnitude and can destroy circuit components employed in the goggles.
It is very difficult to harden against electromagnetic interference when the battery pack is subjected to electromagnetic fields of 200 V/M or more. Normally two approaches are used: (1) Faraday Shield the entire goggle system; or (2) shunt all EMI signals to ground. The first approach, Faraday shielding, is not feasible. Even if the conductors were shielded, there would be some leakage in the shield and, as a result, the first, second and third conductors 5, 6 and 7 would pick up some RF signals. Furthermore, Faraday shielding the entire goggle system would require a major redesign of the entire system. The second approach, shunting to ground, is also not feasible since the battery pack does not have an effective ground plane. Any attempt to shunt EMI signals to ground would fail because a ground leg conductor would be as hot (EMI wise) as the other conductors 5, 6 and 7. Thus, filtering does not work very well at all.
Therefore, it is an object of the present invention to EMI harden the battery pack against RF signals without having to redesign the entire night vision goggle system.